Micro Electro-Mechanical System (MEMS) sensor devices, including accelerometers, based on capacitive pick-off and electrostatic closed-loop rebalance are generally well known.
FIG. 1 illustrates in accordance with prior art, a capacitive pick-off MEMS sensor constructed as a conventional mid-pendulum hinged or “teeter-totter” type. Such devices are constructed using microcircuit techniques to produce reliable, maintenance-free capacitive acceleration-sensing devices. Such a capacitive acceleration sensing device 1, hereinafter a capacitive accelerometer, includes a pair of stationary substrates 2, 3 having opposed parallel planar faces. The substrates 2, 3 are spaced from one another and each has a number of metal electrode layers 4, 5 of predetermined configuration deposited on one surface to form respective capacitor electrodes or “plates.” The electrode elements 4 (or 5) operates as an excitation electrode to receive stimulating signals,and the other electrode elements 5 (or 4) operate as the feedback electrodes for electrostatic rebalance. A single set of electrode elements 4 (or 5) operates as both excitation and feedback electrodes when the feedback signal is superimposed on the excitation signal.
A pendulous acceleration sensing element 7, which operates as pick-off electrode, is flexibly suspended for pendulous rotation about a hinge axis H to form different sets of capacitors with electrode elements 4, 5. Movement of the acceleration-sensing element, or “pendulum,” 7 in response to acceleration changes its position relative to the stationary excitation electrodes 4 (or 5), thereby causing a change in pick-off capacitance. This change in pick-off capacitance is indicative of acceleration. A set of capacitors for electrostatic rebalance is made up of the sensing element 7 and the feedback electrodes 5 (or 4) for driving the sensing element 7 to its reference position and maintaining it there.
In such an acceleration sensor device, a capacitance formed by the excitation electrodes 4 (or 5) and the moveable sensing element 7 is related to 1/D, where D is the separation from stationary substrates 2, 3 to the hinge axis (H) of the pendulous acceleration sensing element 7.
A desirable characteristic of an accelerometer is a linear response for pick-off capacitance C versus acceleration input g. However, conventional MEMS high-g range accelerometers have less than optimum linearity for high performance application and may also have a non-monotonic response for electrostatic rebalance force versus acceleration when feedback voltage is capped. The capacitance seen by the pick-off electrodes is related to the integral of 1/d(i) for each a(i) over the area of the excitation electrodes, where d(i) is the dynamic separation distance between the stationary electrodes and the pendulum for each incremental area a(i). The sensor's dynamic range, scale factor and response linearity are thus defined by the separation distance D (shown in FIG 1) between the stationary electrode elements 4, 5 and the hinge axis of the pendulous acceleration-sensing element 7, and the positions of electrode elements 4, 5 relative to the hinge axis of the pendulous acceleration-sensing element 7. In a conventional MEMS teeter-totter type acceleration sensor device, the stationary capacitor electrodes 4, 5 are traditionally arranged substantially along a longitudinal axis of symmetry L of the acceleration sensing device 1 perpendicular to the hinge axis H of the acceleration-sensing element 7, as illustrated in FIG 1. Electrode elements 4, 5 are sized and spaced symmetrically with respect to the longitudinal axis L of the acceleration sensing device 1, while the electrode elements 4 (or 5) operating as excitation electrodes are further sized and spaced symmetrically with respect to the hinge axis H of the moveable sensing element 7. Therefore, adjustments of the positions and expansion of the areas of electrode elements 4 and 5 are limited to be in the directions indicated by the arrows in FIG. 1. Area and position adjustment of electrode elements 4 in reverse direction can not be accomplished without affecting electrode elements 5, and vice versa. In other words, because the electrode elements 4 and 5 cannot occupy the same area of the substrate, adjustment of one of the electrode elements almost always necessitates adjustment of the other electrode element. As a result, improving response performance presents a challenge to the device designer. For example, improved response linearity and scale factor is generally achieved by sacrificing dynamic range.